The present invention relates to a device for thermally fixing a toner image developed with liquid developer for use in a copying apparatus employing a curved heating plate, along which the copying sheet carrying the toner image developed with liquid developer is guided by a guide roller arranged in the fixing station, and a device for heating the heating plate.
Thermal fixing stations are used in electrophotographic copying apparatuses in which a charge image is produced on a photoconductor layer and developed with developer liquid, to form a toner image by separated toner pigments. The toner image is transferred to a copy support, for example, paper. The copy support carrying the toner image which is still moist with developer is passed into the thermal fixing device where the toner image is to be fixed on the paper by the action of heat, so that it becomes wipe-resistant.
German Offenlegungsschrift No. 25 39 642 (equivalent to U.S. Pat. No. 4,059,394) discloses an apparatus for fixing a toner image by means of heat, in an electrophotographic copier using liquid developer. This fixing apparatus is provided with a heating installation along which the copy sheet developed with liquid developer and carrying the toner image on its upper surface is moved. The heating installation has a curved heating surface. The backside of the copy sheet is in sliding contact with the curved heating surface, in order to fix the toner image by means of heat. A partial area of the heating surface is curved. At a small distance from the heating surface, there is provided a guide roller which is brought into contact with the toner image-carrying surface of the sheet, in order to move the latter to the curved part of the heating surface and to introduce it into that part. The guide roller has a knurled circumferential surface, and its distance from the curved part of the heating surface is greater than the thickness of the copy sheet.
Liquid toners usually comprise dielectric liquids, such as Isopar G, an isoparaffinic hydrocarbon having a boiling point of about 164.degree. C., in which charged pigments, preferably carbon black pigments, are dispersed. The liquid toners furthermore contain admixtures which control the charge of the pigments, binders, and if appropriate, fusible waxes having melting temperatures of about 80.degree. C. When the toner image is transferred to the copy support, for example, paper or film, the support is also moistened with dispersing liquid. This transferred dispersing liquid is evaporated in the fixing device by the action of heat, and in the case of wax-containing toners, fusing of the separated-off wax particles is desirable. For this purpose, the copy support, which has been treated with toner, is moved, in known manner, (German Offenlegungsschrift No. 25 39 642) over a heating plate which normally has a temperature of between about 180.degree. C. and 210.degree. C. The higher temperature range is preferably chosen for wax-containing toners.
Fixing temperatures exceeding the above values are not used, because an unpredictable jamming of the copying material would result in the paper turning yellow, scaling, or, in the case of films, melting. When the copying apparatus is turned on, the known heating plate is heated during a relatively long warm-up period, and in intervals where no copies are made, the heating plate cools down again. When the copying apparatus is turned on again after a longer standstill, either the resulting copies are not properly fixed or a relatively long warming-up time which can last several minutes, must be accepted before the optimum final temperature is reached and copies fixed in a wipe-resistant manner can be obtained.